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Tailoring the defects and carrier density for beyond 10% efficient CZTSe thin film solar cells
Solar Energy Materials & Solar Cells 159 (2017) 447–455
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Tailoring the defects and carrier density for beyond 10% efficient CZTSe thin film solar cells
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Jianjun Lia, SeongYeon Kimb, Dahyun Namc, Xiaoru Liua, JunHo Kimb, Hyeonsik Cheongc, ⁎ Wei Liua, Hui Lid, Yun Suna, Yi Zhanga, a
Institute of Photoelectronic Thin Film Devices and Technology, Nankai University, Tianjin 300071, PR China Department of Physics, Incheon National University, 119 Academy-ro, Yeonsu-gu, Incheon 22012, Republic of Korea c Department of Physics, Sogang University, Seoul 04107, Republic of Korea d The Key Laboratory of Solar Thermal Energy and Photovoltaic System, Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, PR China b
A R T I C L E I N F O
A BS T RAC T
Keywords: CZTSe solar cell Defects states Carrier density Admittance spectroscopy Ratio of Zn/Sn
The defects states and carrier density of CZTSe absorber layers are two of the crucial factors that decide the photovoltaic performance of CZTSe thin film solar cells. Fine tailoring the defects and carrier density is a key to push the power conversion efficiency of CZTSe solar cells to a more competitive level. In this work, the phase properties, defect states, and carrier density of CZTSe thin film are well controlled by fine tuning the ratio of Zn/ Sn in the range from 0.75 to 1.27. Capacity-Voltage measurements and Admittance Spectroscopy are used to characterize the carrier density, depletion region width, and defect states of the CZTSe solar cells. The results indicate that the defects states and carrier density of CZTSe layer are very sensitive to the ratio of Zn/Sn. Combining experimental results and numerical simulation, the statistic regularities of the photovoltaic parameters of the CZTSe solar cells with different ratios of Zn/Sn is well explained. The increase of VOC of CZTSe solar cells with the ratio of Zn/Sn is related to both the increased carrier density and the decreased deep level defects states. The decline of JSC of the Zn-rich solar cells is caused by both the shrunken depletion region width and a large barrier caused by ZnSe phase. This barrier is the cause for a low fill factor in the Zn-rich solar cells. Overall, the CZTSe solar cells with a stoichiometric ratio of Zn/Sn=1.02 have favorable defects property and carrier density, thus resulting in the highest photovoltaic efficiency of 10.21%.
1. Introduction Kesterite based Cu2ZnSnSe4 (CZTSe) and Cu2ZnSnS4 (CZTS) materials are promising light absorber materials for thin film solar cells due to low cost, low-toxicity processing techniques, and large potential for high photovoltaic performance [1–3]. Up-to-date, CZTSSe solar cells have achieved encouraging 12.6% efficiency [4]. However, this efficiency is still far away from its predecessor Cu(In, Ga)Se2 (CIGS) solar cells, which have already achieved 22.6% efficiency [5]. Comprehensive understanding of the defects chemistry of kesterite materials is one of the keys to push the power conversion efficiency of CZTSSe solar cells to a more competitive level. The results of first principle calculation on kesterite materials conducted by Chen et al. have revealed that under Cu-poor and Zn-rich conditions the dominant lattice defects are CuZn, VCu, and ZnCu, and the dominant charge compensated defects clusters is [VCu+ZnCu] [6]. These defects and defects clusters are benign in kesterite solar cells [2,3,6,7]. Both experimental results and theoretical results have disclosed that the ⁎
optimal ratio of Zn/Sn for CZTSSe solar cells with high S content is about 1.2–1.3 [1,6–9]. Recent results show that high performance pure selenide CZTSe solar cells adopt lower ratio of Zn/Sn than that of CZTSSe solar cells with high S content. For instance, the record 11.6% efficiency CZTSe solar cell by evaporation approach has the ratio of Zn/ Sn=1.04 [10]. Brazmmertz et al. also fabricated 9.7% CZTSe solar cell with Zn/Sn=1 by DC sputtering and post annealing process [11]. Recently, we have fabricated 10.4% CZTSe solar cell based on the ratio of Zn/Sn=1.03 by sputtering and post annealing process [12]. However, the underlying mechanism is unclear. The possible reasons may be related with the secondary phases, lattice defects, defects clusters and carrier concentration of the CZTSe system. In order to comprehensively understand how the ratio of Zn/ Sn influences the secondary phases, lattice defects, defects clusters, and carrier concentration of the CZTSe system, thus influencing the performance of CZTSe solar cell, and give an optimal ratio of Zn/Sn for high performance pure selenide CZTSe solar cells, in this work we have systematically studied the secondary phases, defects, carrier
Corresponding author.
http://dx.doi.org/10.1016/j.solmat.2016.09.034 Received 5 August 2016; Received in revised form 16 September 2016; Accepted 22 September 2016 0927-0248/ © 2016 Elsevier B.V. All rights reserved.
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stylus profiler. The composition of the precursor films was determined by a Magix PW2403 X-ray Fluorescent spectrometer (XRF) with a Rhanode, which was precisely calibrated using inductively coupled plasma spectroscopy to ensure its accuracy. The background signal from Mocoated glass substrates had been subtracted. The structural properties of the precursors and the selenized samples were analyzed by a Philips X-pert pro X-ray diffractometer (XRD) with Cu K-α as the radiation with a continuous scan in 4 min for 2-theta scanning from 10 to 90 degree. A Raman spectrometer TRIAX 550 was used to analyze the phase composition of the selenized samples, using 20 mW line-focused 441.6 nm (He-Cd) laser. A scanning electron microscope (SEM, Zeiss ΣIGMA) was used to characterize the morphology of the films and devices. The Transmission Election Microscope (TEM) measurement was carried out using Tecnai G2 F20 S-TWIN (200 kV) coupled with Energy Dispersive Spectroscopy (EDS). The current density–voltage (JV) characteristics of the CZTSe solar cells were measured by a solar simulator under the standard AM1.5 spectrum with an illumination intensity of 1000 W m−2 using a Keithley 2420 source meter unit at room temperature. The light intensity of the solar simulator was calibrated with a standard monocrystalline Si reference solar cell. External quantum efficiency (EQE) measurements were performed by measuring the short-circuit current with spectrally resolved monochromatic beam and locked-in amplifier, using calibrated Si and InGaAs photodiodes as references. The capacitance-voltage (C-V) characterizations were performed using an Agilent B 1500 A apparatus. The C-V data were measured by using 50 mV and 100 kHz alternating current (AC) excitation source with direct current (DC) bias from 0.5 to −1.0 V at room temperature. Admittance spectroscopy measurements were carried out in the temperature of 90–300 K under dark condition with a LCR meter (E4980A, Agilent), which applied an AC voltage of 30 mV by varying the frequencies from 20 Hz to 2 MHz. We obtained all capacitance spectra at corresponding temperatures with a temperature error of ± 0.05 K or less.
density and device performance of CZTSe solar cells with different Zn content in a wide range of Zn/Sn≈0.75 to Zn/Sn≈1.27. The results in this work clearly show that the secondary phases, defects, and carrier density of CZTSe solar cells can be tailored by fine tuning the ratio of Zn/Sn. The admittance spectroscopy results disclose that lots of deep level defects exist in CZTSe solar cells as Zn/Sn < 1; while deep level defects decrease dramatically and shallow acceptor defects such as CuZn and VCu increase significantly as Zn/Sn≥1. The carrier density increases almost exponentially with the ratio of Zn/Sn. Based on the comparison of statistics data of the photovoltaic performance parameters, the optimal ratio of Zn/Sn is deduced that it should be exactly near Zn/Sn=1.0. The best device of 10.2% efficiency is fabricated according to the ratio of Zn/Sn=1.02. 2. Experimental 2.1. Preparation of films and solar cells The precursors stacked with the sequence of Mo/Zn/Cu/Sn/Cu were deposited by DC-magnetron sputtering on Mo-coated soda lime glasses using metal targets with 99.99% purity. The DC power for the Cu and Sn targets is 0.390 and 0.415 W/cm2 respectively. The deposition rate for both Cu and Sn layer is about 25 nm/min. The thickness of Sn layer is about 370 nm. The sputtering process of the two Cu layers is the same, and the total thickness of the two Cu layers is about 240 nm. The ratio of Cu/Sn of all the samples is fixed to about 1.70. The thickness and deposition rate of Zn layer varies by adjusting the DC power to achieve different ratios of Zn/Sn. The as-sputtered precursors were firstly annealed at 300 °C for 20 min in 104 Pa Ar atmosphere. After that, the temperature was raised to 550 °C with a heating rate of 40 °C/min and remained at 550 °C for 15 min in Se and Ar mixed atmosphere. The Ar atmosphere keeps still during the whole annealing process. A Knudsen-type effusion cell with a 1.5 mm spout was used as the Se source. The Se source was filled with excessive solid Se before selenization process. A hot molybdenum barrel (length=12 cm, inner diameter=1.5 cm) was used between the Se source and the substrates to thermally crack the macromolecule Se vapour (Se6, Se5, and Se7) into micro-molecule Se vapour (Se2). The distance between the Mo barrel and the substrates is about 10 cm. There was little influence of the hot Mo barrel on the substrate temperature because there was a heat shield at the outside of Mo barrel. During the temperature rising stage after alloying process at 300 °C, the Se source temperature increased to 300 °C, and the cracking barrel temperature was 800 °C, while during the hightemperature selenization at 550 °C, the Se source temperature changed to 450 °C in 1 min, and the cracking barrel temperature changed to 500 °C in 1 min. More details about the selenization process are explained in our previous work [12,13]. All the samples in this work were prepared using the same selenization process. The CZTSe thin film solar cells were fabricated by deposition of a 50 nm CdS buffer layer at 83 °C for 12 min by a chemical bath deposition method (CBD) on top of the prepared CZTSe absorber film, followed by deposition of a 50 nm i-ZnO layer and a 500 nm ZnO:Al layer. The i-ZnO layer was deposited by medium frequency sputtering process using an intrinsic ZnO target in 0.4 Pa pure Ar atmosphere. The ZnO:Al layer was deposited by DC-sputtering using an Al doped ZnO target (with Al 2 wt%) in 0.4 Pa Ar atmosphere. Then Ni/Al grid contacts were deposited by electron beam evaporation. The total area and active area of each CZTSe solar cell are about 0.468 cm2 and 0.345 cm2, respectively, determined by mechanical scribing and an optical microscope. No etching was used in the device fabrication in this work.
3. Results and discussion 3.1. Material structure and secondary phases The compositions of the precursors and the selenized CZTSe layers are listed in Table 1. The ratio of Cu/Sn of all the samples is about 1.70, while the ratio of Zn/Sn for the 7 group samples varies from about 0.75-1.27. There are 3-4 samples in each group. Fig. 1(a) presents the XRD patterns of the CZTSe films with different ratios of Zn/Sn. All the samples show 112, 220, and 312 strong diffraction peaks of CZTSe. As shown in Fig. 1(b), the intensities of the 112 peak for all the samples are beyond 3×104 counts，suggesting good crystallization of all the Table 1 The mean compositions of the precursors and the CZTSe films measured by XRF, and the numbers of samples for each group, where M=Cu+Zn+Sn. Group
2.2. Characterization The thickness of the films was determined by an AMBIOS XP-2 448
Number of samples
#1
3
#2
3
#3
3
#4
3
#5
4
#6
4
#7
3
Precursor
CZTSe
Cu/Sn
Zn/Sn
Cu/Sn
Zn/Sn
Se/M
1.69 ± 0.04 1.70 ± 0.02 1.71 ± 0.03 1.70 ± 0.03 1.70 ± 0.03 1.71 ± 0.04 1.68 ± 0.04
0.74 ± 0.03 0.83 ± 0.04 0.91 ± 0.03 1.01 ± 0.03 1.05 ± 0.03 1.13 ± 0.04 1.25 ± 0.05
1.72 ± 0.05 1.71 ± 0.04 1.72 ± 0.03 1.70 ± 0.02 1.70 ± 0.02 1.71 ± 0.03 1.68 ± 0.05
0.75 ± 0.05 0.84 ± 0.04 0.92 ± 0.04 1.02 ± 0.03 1.07 ± 0.04 1.15 ± 0.03 1.27 ± 0.04
1.03 ± 0.04 1.03 ± 0.03 1.03 ± 0.02 1.02 ± 0.03 1.03 ± 0.02 1.03 ± 0.03 1.04 ± 0.03
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Raman spectroscopy with 441.6 nm He-Cd lasers was used to distinguish the ZnSe secondary phase which is difficult to be identified by XRD measurement. Because of the large absorption coefficient of CZTSe material [1], during Raman measurement the penetration depth of the 441.6 nm laser is only 100–200 nm. Therefore, the results of Raman measurements only show the information of surface phase composition. The Raman spectra of the CZTSe films with different ratios of Zn/Sn are shown in Fig. 2. The Raman peaks around 170 cm−1, 196 cm−1, and 230 cm−1 belong to the B mode, A mode, and E mode of CZTSe, respectively, while the Raman peak around 250 cm−1 belongs to the main Raman peak of ZnSe [14,15]. The ZnSe signal can be identified in the samples with Zn/Sn > 1, and their intensities increase as the ratio of Zn/Sn increases. These results indicate that the ZnSe secondary phase should exist on the surface of these samples. Meaning while, weak signals near 140 cm−1 belonging to SnSe are found in these samples with Zn/Sn < 1 [16], thus identifying the existence of small amount of SnSe secondary phase in these samples. Combining above results, and considering that the 112 diffraction peaks of Zn-rich samples should also have contributions of ZnSe phase, the decrease of 112 peak intensity indicates the crystallinity is getting worse when the ratio of Zn/Sn is larger than 1.0. 3.2. Comparison of device performances In order to obtain convincing statistics data of the photovoltaic performance of the CZTSe solar cells with different ratios of Zn/Sn for comparison, we have fabricated more than 30 solar cells from 3 to 4 samples for each group with the same ratio of Zn/Sn. Each 4×4 cm2 sample produces 16 solar cells. However, because of the edge effect, the performance of 4–6 solar cells located at the four corners of substrates is always very bad compared to the other solar cells of this sample. Therefore, we omitted these solar cells from each sample during the statistics. More solar cells with Zn-rich CZTSe films were fabricated than those with Zn-poor CZTSe films to verify if Zn/Sn=1.2–1.3 is the optimal ratio for the high-performance CZTSe solar cells. The selenization process and the device fabrication processes of all the solar cells are the same. The cross section images of the CZTSe solar cells with different ratios of Zn/Sn are shown in Fig. 3. The absorbers mainly consist of large and compact CZTSe crystals on the top. The thickness of the large grains layer is around 2 µm. There are also fine grains at the bottom of CZTSe layer. Fig. 4(a) and (b) present the J-V and EQE data of the representative CZTSe solar cells with different ratios of Zn/Sn in the absorber layer respectively. The detailed photovoltaic parameters of these solar cells are listed in Table S1. Obviously, the open circuit Voltage (VOC) of these solar cells increases with the ratio of Zn/Sn, as shown in Fig. 4(a). The VOC of the CZTSe solar cells with Zn-poor absorber is less than 400 mV, while the VOC of the solar cell with Zn/ Sn=1.02 reaches 424 mV, and the Zn-rich solar cells have even higher VOC. The highest VOC of the CZTSe solar cell reaches 478 mV with the ratio of Zn/Sn=1.17. However, the VOC of the cell with the highest ratio of Zn/Sn=1.27 drops to 412 mV. On the other hand, the short-circuit current density (JSC) of the solar cells with Zn/Sn≤1 is larger than that of the solar cells with Zn/Sn > 1, which is also confirmed by the EQE data of these solar cells (Fig. 4(b)). As shown in Fig. 4(c), the band gaps of the absorber layers derived from these EQE data are in a narrow range from 1.010 to 1.045 eV. It shows the band gaps of the CZTSe layers with Zn/Sn < 1 are close to 1.01 eV, while the band gaps of the CZTSe layers with Zn/Sn > 1 are close to 1.04 eV. The small variation of band gap may result from the variation of lattice constants of CZTSe as the ratio of Zn/Sn increases [17]. The increased band gap of the Znrich CZTSe solar cells may contribute to the higher VOC of these solar cells. However, the maximum increment of band gap is only 35 meV, which is much smaller than the maximum increment of VOC (138 mV). It indicates that the variation of band gap is not the dominant factor that influences the VOC of CZTSe solar cells. The statistics data of efficiency, VOC, JSC, and fill factor are shown in
Fig. 1. (a): The XRD patterns of the CZTSe films with different ratios of Zn/Sn, and (b): The intensity of 112 peaks and the ratio of 112/220 intensity of the CZTSe films with different ratios of Zn/Sn.
Fig. 2. The Raman spectra of the CZTSe films with different ratios of Zn/Sn. The intensities are normalized to the peaks of 196 cm−1. The red arrow indicates the changes of the intensities of B mode of CZTSe, while the blue arrows indicate the change of the intensities of E mode and Sn-Se like mode of CZTSe as the ratio of Zn/Sn increases.
CZTSe films. The intensity of 112 diffraction peaks decreases slightly with the increase of Zn/Sn when the ratio of Zn/Sn is over 0.92. However, the ratio of 112/220 intensity does not change obviously with the ratio of Zn/Sn, indicating the ratio of Zn/Sn does not have a significant influence on the preferred orientation of the CZTSe films. 449
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Fig. 3. (a), (b),(c),(d),(e),(f): The cross section SEM images of the CZTSe solar cells with different ratios of Zn/Sn in absorber layer. (1) (1) (2) is the VBO of ZnSe/ ΔEVBM = ΔEVBM + ΔEVBM = 0.78 eV, where ΔEVBM (2) CuInSe2, and ΔEVBM is the VBO of CuInSe2/CZTSe. These results indicate that both the conduction band offset and valence band offset between ZnSe and CZTSe are too large that ZnSe phase can severely block both the transport of the electron current from CZTSe absorber layer to ZnO front contact and the hole current from CZTSe absorber layer to Mo back contact. TEM and EDS measurements were used to locate the position and determine the size of ZnSe clusters in the Znrich solar cells. Fig. 6 presents the TEM image of the cross-section of the solar cell with Zn/Sn=1.27. The EDS spectra of the regions labeled by red numbers are shown in Fig. S1, and the atomic percentage of the elements of the corresponding EDS spectrum is listed in Table S3. The results indicate that the large crystals near the ZnO layer (region 1) are CZTSe (region 2), and the thin layer near Mo layer is MoSex (region 4). Large ZnSe clusters with a size of about 200 nm locate at the region near the back contact (region 3). There are also lots of small grains and voids in this region. Combining with the results of surface Raman measurements, it is indicated that the poor fill factor of Zn-rich CZTSe solar cells should be attributed to the ZnSe secondary phase at the surface and the bottom of CZTSe layer. The large barrier caused by ZnSe phase may also deteriorate the JSC of the Zn-rich CZTSe solar cells.
Fig. 5(a), (b), (c), and (d), respectively. The mean value and standard deviation of the photovoltaic parameters of these solar cells are listed in Table S2. Fig. 5(a) clearly shows that the mean efficiency of the solar cells increases with the increase of the ratio of Zn/Sn at the Zn-poor region. However, the mean efficiency decreases dramatically at the Znrich region. As the ratio of Zn/Sn is 1.02, the CZTSe solar cells achieve the highest mean efficiency of 9.11%. However, the solar cells with the highest mean VOC (438 mV) are fabricated based on the absorber with Zn/Sn=1.07, as shown in Fig. 5(b). The average JSC is fundamentally invariant at the Zn-poor region and at Zn/Sn=1.02, while it decreases rapidly with the increase of Zn content at the Zn-rich region. The variation of fill factor is basically similar with that of efficiency. The poor fill factor of the Zn-poor CZTSe solar cells should be attributed to the SnSe secondary phase on the surface of CZTSe layer as shown by the results of Raman measurements. The highest mean fill factor is 63.4% at Zn/Sn=1.02, while the fill factor drops dramatically at the Znrich region. Thus the mean fill factor of the CZTSe solar cell with Zn/ Sn=1.27 is only 20.4%, leading to poor efficiencies. It should be noted that all the photovoltaic parameters of the Zn-rich solar cells show larger standard deviation than those of the solar cells with Zn/Sn≤1.02, which implies that the uniformity and reproducibility becomes worse for the Zn-rich samples. The decrease of fill factor of the Zn-rich solar cells may be caused by the high resistant ZnSe phase at the surface or the bottom of CZTSe absorber. EBIC measurement and quantification analysis suggested that the surface ZnSe acts as a current blocking phase in CZTSe solar cells [18,19]. Furtherly, we can primarily analyze the transport of electron and hole current from CZTSe layer to ZnSe phase by studying the conduction band offset (CBO) and valence band offset (VBO) of ZnSe/CZTSe interface. First-principles calculations on band structure and in suit photoemission measurements have revealed that the CBO and VBO of ZnSe/CuInSe2 hetero-junction are 0.90 eV and 0.70 eV respectively [20,21]. The band structure calculation by Chen et al. suggested that the CBO and VBO of CuInSe2/CZTSe are −0.04 eV and +0.08 eV, respectively [22]. Therefore, the CBO of ZnSe/CZTSe inter(1) (2) (1) face is given by ΔECBM = ΔECBM is the + ΔECBM = 0.86 eV, where ΔECBM (2) CBO of ZnSe/CuInSe2, and ΔECBM is the CBO of CuInSe2/CZTSe; the VBO of ZnSe/CZTSe interface is given by
3.3. Carrier density and simulations C-V measurements are carried out to characterize the carrier density and depletion region width of the CZTSe solar cells. Because a relatively high frequency AC excitation (100 kHz) was used during CV measurements for CZTSe solar cells, the defects and interface states with low escaping frequency does not respond to the high frequency excitation [23,24], and the carrier densities from C-V measurements at 0 V bias mainly represent the responses of free carriers. As shown in our previous work, the carrier densities measured from C-V and drive level capacity voltage (DLCP) measurements are very close when 100 kHz AC excitation was used during the measurements [25]. Fig. 7(a) shows the plots of carrier density (NCV) vs. distance to the junction interface, which are extracted from C-V data. Fig. 7(b) presents the statistics data of the carrier density and depletion region 450
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respectively. The QE and JSC decrease as NA increases, which should be attributed to the shrunken depletion region width as shown in the simulated band diagrams of these solar cells (Figure S2). Because the minority carrier life times (τ) of the CZTSe solar cells fabricated by the same sputtering and selenization processes in this work are in a small range of 1–2 ns [12], the minority carrier life time should have less influence on the collection of photo-generated carriers than the drastic changed depletion region width. However, the VOC increases with NA. As shown in Figure S2, the quasi-fermi level of holes (Efp) of these solar cells moves toward EV of CZTSe layer as NA increases. Therefore, the value of Efn- Efp increases with NA, which facilitates the charge separation and leads to the increase of VOC. However, the narrow depletion region width caused by the too high carrier density may lead to severe tunnel enhanced interface recombination, thus deteriorating the VOC of the device. This may be the reason why the average VOC decreases when the ratio of Zn/Sn is higher than 1.07. The simulated photovoltaic parameters for these three solar cells are listed in Table 2. The solar cell with moderate NA (1×1016 cm−3) achieves best efficiency, which is in good accordance with the experimental statistics results in Fig. 5(a). 3.4. Defects and defects clusters According to the study by Dimitrievska et al., the changes in intensities of some modes of Raman spectra reveal the information about the population changes of most dominant charge compensated defect clusters in CZTSe, namely, [VCu+ZnCu] and [2CuZn+SnZn] [27]. As shown in Fig. 2, the intensity of B modes around 170 cm−1 with respect to the A mode at 196 cm−1 decreases obviously as the ratio of Zn/Sn increases, indicating the decrease of Cu/Zn and Cu/Sn vibration units [17,27]. The decrease of B mode intensity implies that the concentration of [VCu+ZnCu] defect clusters increases with the ratio of Zn/Sn. Meanwhile, the relative intensities around 185 cm−1 and 230 cm−1 (E mode) also decrease pronouncedly with the increase of Zn/Sn ratio. The decreased intensity of Raman spectra around 185 cm−1 indicates the decreased concentration of Sn-Se vibration unites, while the decreased intensity at the region around 230 cm−1 implies the decrease of SnZn anti-sites [27]. Though the formation energy of SnZn point defect is very high, by forming [2CuZn+SnZn] defect clusters which have very low formation energy, the possibility of existence for SnZn anti-sites will increase significantly, especially in a Sn-rich and Zn-poor composition [6]. In summary, according to the changes in the intensities of B mode around 170 cm−1 and E mode around 230 cm−1, it is revealed that the population of benign [VCu+ZnCu] defect clusters increases and the population of detrimental [2CuZn+SnZn] defect clusters decreases as the ratio of Zn/Sn increases in the range from 0.75 to about 1.27. The high relative intensities of E mode around 230 cm−1 of the Zn-poor samples with Zn/Sn=0.76 and 0.84 imply that high population of [2CuZn+SnZn] defect clusters may exist in these samples. Generally speaking, these variations of [VCu+ZnCu] and [2CuZn+SnZn] defect clusters are beneficial to reduce the band tail states and improve the VOC [6,28]. Admittance spectroscopy (AS) measurements were carried out to investigate the impact of the ratio of Zn/Sn on the defect energy level and the defect density in the CZTSe solar cells. The temperaturedependent capacity frequency (Cf-T) spectra (admittance spectra) in a temperature range from 90 K to 300 K of three solar cells with Zn/ Sn=0.78 (Cell-A), 1.03 (Cell-B), and 1.12 (Cell-C) are shown in Fig. 9(a), (b), and (c), respectively. The composition of CZTSe films and the photovoltaic parameters of these solar cells are listed in Table 3. The Zn-rich Cell-C has the highest VOC but the lowest JSC and FF. Cell-B which is with Zn/Sn=1.03 has the moderate VOC and highest JSC and FF, achieving the highest efficiency among these three cells. The Arrhenius plots for the inflection points in the admittance spectra of three cells at different temperature are shown in Fig. 10. The inflection point frequency ω0 for each C-f-T curve is determined from
Fig. 4. The J-V curves (a) and EQE responses (b) of the representative CZTSe solar cells with different ratios of Zn/Sn. (c): The band gaps of the CZTSe layers derived from the EQE data in (b).
width of the solar cells with different ratios of Zn/Sn. Obviously, the carrier density increases with the ratio of Zn/Sn in the whole region from Zn/Sn=0.75 to Zn/Sn=1.27, which consolidates the conclusion of our previous work [25]. In the Zn-poor region, the carrier density is in the range of 1×1015 cm−3–5×1015 cm−3, while in the Zn-rich region, the carrier density is in the range of 1×1016 cm−3–1×1017 cm−3. In accordance, the depletion region of the solar cells shrinks as the carrier concentration increases. The average depletion region width of the solar cells decreases from about 500–60 nm as the ratio of Zn/Sn increases from 0.75 to 1.27. The effects of free carrier concentrations on the performances of CZTSe solar cells have been simulated using the software wxAMPS [26]. The setting of the simulation is listed in Table S4. The simulated J-V and QE data of three CZTSe solar cells with NA=5×1015 cm−3, NA=1×1016 cm−3, and NA=5×1016 cm−3 are shown in Fig. 8(a) and (b), 451
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Fig. 5. The statistics data of efficiency (a), VOC (b), JSC (c), and fill factor (d) of the CZTSe solar cells with different ratios of Zn/Sn. The mean value and deviation of these data are listed in Supporting information Table S2.
During AS measurements, the deep level defects with larger activation energy will be dielectric frozen out before the shallow defects when the temperature drops to a certain value. Therefore, the Ea of deep level defects is fitted at the relatively high-temperature region, while the Ea of shallow defects is fitted at the relatively low-temperature region. The defect energy levels of the Zn-poor sample Cell-A are 123 meV and 243 meV, while the trap energy level of the Cell-B and Cell-C are 51 meV and 140 meV, 47 meV and 150 meV, respectively. According to the first principle calculation, the intrinsic point defects and charge-compensated defect clusters with lowest formation energy in the Cu-poor CZTSe equilibrium are CuZn, [2CuZn+SnZn], and [VCu+ZnCu] [3,6]. The defects with activation energy between 120 meV and 150 meV present in all the samples with different ratios of Zn/Sn. Based on the theoretical study and experimental results, these traps with activation energy in the range of 120–150 meV can be assigned as the dominant acceptor impurities CuZn anti-sites [7,29]. Regarding the formation energy and excitation energy, the shallow traps with the energy level of 51 meV and 47 meV in Cell-B and Cell-C should be attributed to VCu, which becomes the dominant free hole source when CuZn anti-sites were dielectric frozen out at the temperature region lower than 150 K. According to the model described by Kimerling [30], the capacity at high frequency is attributed to the response of free carrier density, while the capacity at low frequency is attributed to the response of the sum of free carrier density and deep traps. The distribution of the deep traps in the band gap follows the equations [23,24]:
Fig. 6. The TEM image of the cross-section of a solar cell with Zn/Sn=1.27. The regions labeled by red numbers were measured by EDS. The yellow dashed circle indicates the boundary of ZnSe nano-particle. The EDS spectra are shown in Supporting information Fig. S1. And the quantitative results are listed in Table S3.
theω at the maximum point of ωdC / dω vs. ω plots. The Arrhenius plots are linearly fitted according to the equation [23]:
⎛ −E ⎞ ω0 = 2πν0T 2 exp⎜ a ⎟ ⎝ kT ⎠
⎛ 2πν T 2 ⎞ 0 ⎟⎟ E (ω) = kT ln⎜⎜ ⎝ ω ⎠
(1)
where ω0 is the inflection point frequency, ν0 is the average escaping frequency of the traps, which is independent of temperature, Ea is the average energetic depth of the defects relative to the valence band maximum, which represents an average defect energy level in the band gap.
Nt (E (ω)) = −
Vd dC ω . . qω dω kT
(2)
(3)
where E is the energy between the trap energy and the top of the 452
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Fig. 7. (a): The plot of free carrier concentration vs. the distance to the junction for representative solar cells with different ratios of Zn/Sn. (b): The statistics of free carrier concentration and corresponding depletion region width of five solar cells for each ratio of Zn/Sn.
Fig. 8. The simulated J-V curves (a) and QE data (b) of three solar cells with different free carrier densities (NA) in CZTSe layer. Table 2 The simulated photovoltaic parameters of the solar cells with different free carrier density.
valence band, ω is the frequency, ν0 is the average escaping frequency of the traps, Vd is the contact voltage of the p-n junction which can be derived from C-V measurement, and k is the Boltzmann constant. The defect density spectra of Cell-A, Cell-B, and Cell-C which are the overlaps of the plots of Nt vs. E at each temperature are shown in Fig. 11(a), (b), and (c), respectively. The densities of the defects in the Zn-rich sample Cell-C are around the order of 1016 cm−3, while those in Cell-A and Cell-B are around 1015 cm−3. It is indicated that the Zn-rich CZTSe solar cells possess a much higher concentration of VCu and CuZn defects which act as the dominant free hole carrier sources. The shallow acceptor VCu is absent in the Zn-poor CZTSe solar cells. Instead, a deep defect level with large excitation energy emerges in the Zn-poor CZTSe solar cells. One may assign the deep level defects with excitation energy of 244 meV in the Zn-poor solar cell as VZn, which is in good accordance with the calculated result (Ea=250 meV) [3,7]. However, the formation energy of VZn point defects and VZn related defect clusters are too high to achieve a large population [6]. The research by Wei et al. also reported a value of 240 meV for the Ea of CZTSSe solar cell (Eg=1.2 eV) [31]. Their experimental and simulation results reveal that the large Ea may be determined by the Fermi-level, which is pinned by combined effects of high concentration shallow acceptors such as CuZn and deep donors such as SnCu and SnZn. Though, the formation energy of SnZn point defect is very large, the [2CuZn+SnZn] defect clusters have a large possibility to exist in Zn-poor samples, which should be attributed to the very low formation energy in Zn-poor equilibrium. It is also consolidated by the previous Raman analysis about the relatively high intensity of E mode in Zn-poor samples. The [2CuZn+SnZn] defect clusters will lower the effective conduction band and act as deep level electron traps in the devices [6], which also contribute to the low VOC of
NA (cm−3) 15
5×10 1×1016 5×1016
PCE (%)
VOC (mV)
JSC (mA/cm2)
FF (%)
8.83 9.14 8.02
363 381 415
40.5 37.8 30.4
59.9 63.5 63.4
the Zn-poor solar cells besides the low free carrier concentration. The defect energy levels of Cell-B and Cell-C are very close. Therefore, the higher VOC of the Zn-rich solar cells than the solar cells with Zn/Sn close to 1 is mainly caused by the increased free carrier concentration. 3.5. The champion solar cell Fig. 12(a) and (b) show the J-V and EQE data of the 10.21% efficiency champion solar cell in this study. This CZTSe solar cell has excellent photovoltaic parameters: VOC=426 mV, JSC=36.1 mA/cm2, and FF=66.4%. The JSC calculated from the integration of EQE is 36.5 mA/cm2. The VOC of this cell is close to that of the record 11.6% efficiency CZTSe solar cell (423 mV), while the JSC is much lower than that cell (40.6 mA/cm2) [10]. It is mainly caused by the reflection loss because no anti-reflection coating is used in this solar cell, which can be implied by the fluctuant of EQE curve. Another origin of the JSC loss is the parasitic absorption of CdS layer, which is clearly confirmed by the low EQE response at the 400–500 nm. Therefore, reducing the thickness of CdS buffer layer (eg. 25 nm) and adding an anti-reflection coating on the top of ZnO:Al window layer will improve the JSC significantly. The diode parameters including Rs, A, and J0 of this solar cell are fitted using Hegedus’ method as shown in Figure S3 [32]. 453
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Fig. 9. The admittance spectra of Cell-A (a), Cell-B (b), and Cell-C (c) in the temperature range from 90 K to 300 K and frequencies from 100 Hz to 2 MHz.
the theoretical calculated value (1.00 eV) [3].
Table 3 The photovoltaic parameters of the solar cells used for admittance spectroscopy measurement. Solar Cell
Cu/Sn
Zn/Sn
Efficiency (%)
VOC (mV)
JSC (mA/ cm2)
Fill Factor (%)
Cell-A Cell-B Cell-C
1.71 1.72 1.70
0.78 1.03 1.12
7.34 8.38 5.25
368 388 440
33.0 34.8 28.6
60.4 62.4 41.7
4. Conclusions In summary, by fine tuning the ratio of Zn/Sn of CZTSe film, we have tailored the carrier density and the defects of CZTSe solar cells successfully. It is clearly revealed that as the ratio of Zn/Sn increases the hole carrier density increases, the concentration of deep level defects decreases, the concentration of shallow acceptor defects (namely, CuZn and VCu) increases, and the ZnSe secondary phase increases. The increase of the VOC of the solar cells with ratio of Zn/Sn from 0.75 to 1.07 is caused by both the declined hole quasi-fermi level resulting from the increased free hole carrier in CZTSe layer, and the decrease of deep level electron traps caused by [2CuZn+SnZn] defect clusters according to the results of Raman and admittance spectroscopy analysis. The JSC of the CZTSe solar cells with the ratio of Zn/Sn from 1.02 to 1.27 drops dramatically because of the shrunken depletion region width and the large potential barrier caused by the ZnSe phase. This potential barrier is also the main cause for the dramatically collapsed fill factor of the Zn-rich CZTSe solar cells. The statistics data show that the CZTSe solar cells with a stoichiometric ratio of Zn/ Sn=1.02 have the best photovoltaic performance. A 10.21% efficiency solar cell is fabricated based on this ratio of Zn/Sn. Acknowledgements This work was supported by the National Natural Science Foundation of China (61274053, 51372121, 51472239, and 51572132) and YangFan Innovative & Entrepreneurial Research Team Project (2014YT02N037) and by the New & Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy (No. 20123010010130). This research
Fig. 10. The Arrhenius plots of the inflection frequencies for Cell-A, Cell-B, and Cell-C, determined from the derivative of the admittance spectra in Fig. 9.The dashed lines are the linear fit of the Arrhenius plots.
The low diode saturation current (J0=5.8×10−7 A/cm2) and ideal factor (A=1.48) indicates a low concentration of recombination centers in this solar cell. The band gap derived from EQE data is 1.03 eV, very close to
Fig. 11. The defect density spectra derived from admittance spectra in Fig. 9 using Gaussian fitting for Cell-A (a), Cell-B (b), and Cell-C (c).
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Fig. 12. The J-V (a) and EQE (b) data of the champion CZTSe solar cell fabricated based on the ratio of Zn/Sn=1.02. Inset of (b): The band gap extracted from EQE curve.
work was also financially supported by the National Research Foundation of Korea (NRF) funded by the Korean government (NRF2014R1A2A1A11053109). We thank Mr, Qing He and Dr. Fangfang Liu for XRD and XRF measurements. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.solmat.2016.09.034. References [1] D.B. Mitzi, O. Gunawan, T.K. Todorov, K. Wang, S. Guha, The path towards a highperformance solution-processed kesterite solar cell, Sol. Energy Mater. Sol. Cells 95 (2011) 1421–1436. [2] A. Polizzotti, I.L. Repins, R. Noufi, S.H. Wei, D.B. Mitzi, The State and future prospects of kesterite photovoltaics, Energy Environ. Sci. 6 (2013) 3171–3182. [3] A. Walsh, S. Chen, S.H. Wei, X.G. Gong, Kesterite thin-film solar cells: advances in materials modelling of Cu2ZnSnS4, Adv. Energy Mater. 2 (2012) 400–409. [4] W. Wang, M.T. Winkler, O. Gunawan, T. Gokmen, T.K. Todorov, Y. Zhu, D.B. Mitzi, Device characteristics of CZTSSe thin-film solar cells with 12.6% efficiency, Adv. Energy Mater. 4 (2014) 1301465. [5] 〈http://www.pv-tech.org/news/zsw-achieves-world-record-cigs-lab-cell-efficiencyof-22.6?from=timeline & isappinstalled=0〉 [6] S. Chen, A. Walsh, X.-G. Gong, S.-H. Wei, Classification of lattice defects in the kesterite Cu2ZnSnS4 and Cu2ZnSnSe4 earth-abundant solar cell absorbers, Adv. Mater. 25 (2013) 1522–1539. [7] S. Chen, J.-H. Yang, X.G. Gong, A. Walsh, S.-H. Wei, Intrinsic point defects and complexes in the quaternary kesterite semiconductor Cu2ZnSnS4, Phys. Rev. B 81
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Tailoring the defects and carrier density for beyond 10% efficient CZTSe thin film solar cells
crossmark
Jianjun Lia, SeongYeon Kimb, Dahyun Namc, Xiaoru Liua, JunHo Kimb, Hyeonsik Cheongc, ⁎ Wei Liua, Hui Lid, Yun Suna, Yi Zhanga, a
Institute of Photoelectronic Thin Film Devices and Technology, Nankai University, Tianjin 300071, PR China Department of Physics, Incheon National University, 119 Academy-ro, Yeonsu-gu, Incheon 22012, Republic of Korea c Department of Physics, Sogang University, Seoul 04107, Republic of Korea d The Key Laboratory of Solar Thermal Energy and Photovoltaic System, Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, PR China b
A R T I C L E I N F O
A BS T RAC T
Keywords: CZTSe solar cell Defects states Carrier density Admittance spectroscopy Ratio of Zn/Sn
The defects states and carrier density of CZTSe absorber layers are two of the crucial factors that decide the photovoltaic performance of CZTSe thin film solar cells. Fine tailoring the defects and carrier density is a key to push the power conversion efficiency of CZTSe solar cells to a more competitive level. In this work, the phase properties, defect states, and carrier density of CZTSe thin film are well controlled by fine tuning the ratio of Zn/ Sn in the range from 0.75 to 1.27. Capacity-Voltage measurements and Admittance Spectroscopy are used to characterize the carrier density, depletion region width, and defect states of the CZTSe solar cells. The results indicate that the defects states and carrier density of CZTSe layer are very sensitive to the ratio of Zn/Sn. Combining experimental results and numerical simulation, the statistic regularities of the photovoltaic parameters of the CZTSe solar cells with different ratios of Zn/Sn is well explained. The increase of VOC of CZTSe solar cells with the ratio of Zn/Sn is related to both the increased carrier density and the decreased deep level defects states. The decline of JSC of the Zn-rich solar cells is caused by both the shrunken depletion region width and a large barrier caused by ZnSe phase. This barrier is the cause for a low fill factor in the Zn-rich solar cells. Overall, the CZTSe solar cells with a stoichiometric ratio of Zn/Sn=1.02 have favorable defects property and carrier density, thus resulting in the highest photovoltaic efficiency of 10.21%.
1. Introduction Kesterite based Cu2ZnSnSe4 (CZTSe) and Cu2ZnSnS4 (CZTS) materials are promising light absorber materials for thin film solar cells due to low cost, low-toxicity processing techniques, and large potential for high photovoltaic performance [1–3]. Up-to-date, CZTSSe solar cells have achieved encouraging 12.6% efficiency [4]. However, this efficiency is still far away from its predecessor Cu(In, Ga)Se2 (CIGS) solar cells, which have already achieved 22.6% efficiency [5]. Comprehensive understanding of the defects chemistry of kesterite materials is one of the keys to push the power conversion efficiency of CZTSSe solar cells to a more competitive level. The results of first principle calculation on kesterite materials conducted by Chen et al. have revealed that under Cu-poor and Zn-rich conditions the dominant lattice defects are CuZn, VCu, and ZnCu, and the dominant charge compensated defects clusters is [VCu+ZnCu] [6]. These defects and defects clusters are benign in kesterite solar cells [2,3,6,7]. Both experimental results and theoretical results have disclosed that the ⁎
optimal ratio of Zn/Sn for CZTSSe solar cells with high S content is about 1.2–1.3 [1,6–9]. Recent results show that high performance pure selenide CZTSe solar cells adopt lower ratio of Zn/Sn than that of CZTSSe solar cells with high S content. For instance, the record 11.6% efficiency CZTSe solar cell by evaporation approach has the ratio of Zn/ Sn=1.04 [10]. Brazmmertz et al. also fabricated 9.7% CZTSe solar cell with Zn/Sn=1 by DC sputtering and post annealing process [11]. Recently, we have fabricated 10.4% CZTSe solar cell based on the ratio of Zn/Sn=1.03 by sputtering and post annealing process [12]. However, the underlying mechanism is unclear. The possible reasons may be related with the secondary phases, lattice defects, defects clusters and carrier concentration of the CZTSe system. In order to comprehensively understand how the ratio of Zn/ Sn influences the secondary phases, lattice defects, defects clusters, and carrier concentration of the CZTSe system, thus influencing the performance of CZTSe solar cell, and give an optimal ratio of Zn/Sn for high performance pure selenide CZTSe solar cells, in this work we have systematically studied the secondary phases, defects, carrier
Corresponding author.
http://dx.doi.org/10.1016/j.solmat.2016.09.034 Received 5 August 2016; Received in revised form 16 September 2016; Accepted 22 September 2016 0927-0248/ © 2016 Elsevier B.V. All rights reserved.
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stylus profiler. The composition of the precursor films was determined by a Magix PW2403 X-ray Fluorescent spectrometer (XRF) with a Rhanode, which was precisely calibrated using inductively coupled plasma spectroscopy to ensure its accuracy. The background signal from Mocoated glass substrates had been subtracted. The structural properties of the precursors and the selenized samples were analyzed by a Philips X-pert pro X-ray diffractometer (XRD) with Cu K-α as the radiation with a continuous scan in 4 min for 2-theta scanning from 10 to 90 degree. A Raman spectrometer TRIAX 550 was used to analyze the phase composition of the selenized samples, using 20 mW line-focused 441.6 nm (He-Cd) laser. A scanning electron microscope (SEM, Zeiss ΣIGMA) was used to characterize the morphology of the films and devices. The Transmission Election Microscope (TEM) measurement was carried out using Tecnai G2 F20 S-TWIN (200 kV) coupled with Energy Dispersive Spectroscopy (EDS). The current density–voltage (JV) characteristics of the CZTSe solar cells were measured by a solar simulator under the standard AM1.5 spectrum with an illumination intensity of 1000 W m−2 using a Keithley 2420 source meter unit at room temperature. The light intensity of the solar simulator was calibrated with a standard monocrystalline Si reference solar cell. External quantum efficiency (EQE) measurements were performed by measuring the short-circuit current with spectrally resolved monochromatic beam and locked-in amplifier, using calibrated Si and InGaAs photodiodes as references. The capacitance-voltage (C-V) characterizations were performed using an Agilent B 1500 A apparatus. The C-V data were measured by using 50 mV and 100 kHz alternating current (AC) excitation source with direct current (DC) bias from 0.5 to −1.0 V at room temperature. Admittance spectroscopy measurements were carried out in the temperature of 90–300 K under dark condition with a LCR meter (E4980A, Agilent), which applied an AC voltage of 30 mV by varying the frequencies from 20 Hz to 2 MHz. We obtained all capacitance spectra at corresponding temperatures with a temperature error of ± 0.05 K or less.
density and device performance of CZTSe solar cells with different Zn content in a wide range of Zn/Sn≈0.75 to Zn/Sn≈1.27. The results in this work clearly show that the secondary phases, defects, and carrier density of CZTSe solar cells can be tailored by fine tuning the ratio of Zn/Sn. The admittance spectroscopy results disclose that lots of deep level defects exist in CZTSe solar cells as Zn/Sn < 1; while deep level defects decrease dramatically and shallow acceptor defects such as CuZn and VCu increase significantly as Zn/Sn≥1. The carrier density increases almost exponentially with the ratio of Zn/Sn. Based on the comparison of statistics data of the photovoltaic performance parameters, the optimal ratio of Zn/Sn is deduced that it should be exactly near Zn/Sn=1.0. The best device of 10.2% efficiency is fabricated according to the ratio of Zn/Sn=1.02. 2. Experimental 2.1. Preparation of films and solar cells The precursors stacked with the sequence of Mo/Zn/Cu/Sn/Cu were deposited by DC-magnetron sputtering on Mo-coated soda lime glasses using metal targets with 99.99% purity. The DC power for the Cu and Sn targets is 0.390 and 0.415 W/cm2 respectively. The deposition rate for both Cu and Sn layer is about 25 nm/min. The thickness of Sn layer is about 370 nm. The sputtering process of the two Cu layers is the same, and the total thickness of the two Cu layers is about 240 nm. The ratio of Cu/Sn of all the samples is fixed to about 1.70. The thickness and deposition rate of Zn layer varies by adjusting the DC power to achieve different ratios of Zn/Sn. The as-sputtered precursors were firstly annealed at 300 °C for 20 min in 104 Pa Ar atmosphere. After that, the temperature was raised to 550 °C with a heating rate of 40 °C/min and remained at 550 °C for 15 min in Se and Ar mixed atmosphere. The Ar atmosphere keeps still during the whole annealing process. A Knudsen-type effusion cell with a 1.5 mm spout was used as the Se source. The Se source was filled with excessive solid Se before selenization process. A hot molybdenum barrel (length=12 cm, inner diameter=1.5 cm) was used between the Se source and the substrates to thermally crack the macromolecule Se vapour (Se6, Se5, and Se7) into micro-molecule Se vapour (Se2). The distance between the Mo barrel and the substrates is about 10 cm. There was little influence of the hot Mo barrel on the substrate temperature because there was a heat shield at the outside of Mo barrel. During the temperature rising stage after alloying process at 300 °C, the Se source temperature increased to 300 °C, and the cracking barrel temperature was 800 °C, while during the hightemperature selenization at 550 °C, the Se source temperature changed to 450 °C in 1 min, and the cracking barrel temperature changed to 500 °C in 1 min. More details about the selenization process are explained in our previous work [12,13]. All the samples in this work were prepared using the same selenization process. The CZTSe thin film solar cells were fabricated by deposition of a 50 nm CdS buffer layer at 83 °C for 12 min by a chemical bath deposition method (CBD) on top of the prepared CZTSe absorber film, followed by deposition of a 50 nm i-ZnO layer and a 500 nm ZnO:Al layer. The i-ZnO layer was deposited by medium frequency sputtering process using an intrinsic ZnO target in 0.4 Pa pure Ar atmosphere. The ZnO:Al layer was deposited by DC-sputtering using an Al doped ZnO target (with Al 2 wt%) in 0.4 Pa Ar atmosphere. Then Ni/Al grid contacts were deposited by electron beam evaporation. The total area and active area of each CZTSe solar cell are about 0.468 cm2 and 0.345 cm2, respectively, determined by mechanical scribing and an optical microscope. No etching was used in the device fabrication in this work.
3. Results and discussion 3.1. Material structure and secondary phases The compositions of the precursors and the selenized CZTSe layers are listed in Table 1. The ratio of Cu/Sn of all the samples is about 1.70, while the ratio of Zn/Sn for the 7 group samples varies from about 0.75-1.27. There are 3-4 samples in each group. Fig. 1(a) presents the XRD patterns of the CZTSe films with different ratios of Zn/Sn. All the samples show 112, 220, and 312 strong diffraction peaks of CZTSe. As shown in Fig. 1(b), the intensities of the 112 peak for all the samples are beyond 3×104 counts，suggesting good crystallization of all the Table 1 The mean compositions of the precursors and the CZTSe films measured by XRF, and the numbers of samples for each group, where M=Cu+Zn+Sn. Group
2.2. Characterization The thickness of the films was determined by an AMBIOS XP-2 448
Number of samples
#1
3
#2
3
#3
3
#4
3
#5
4
#6
4
#7
3
Precursor
CZTSe
Cu/Sn
Zn/Sn
Cu/Sn
Zn/Sn
Se/M
1.69 ± 0.04 1.70 ± 0.02 1.71 ± 0.03 1.70 ± 0.03 1.70 ± 0.03 1.71 ± 0.04 1.68 ± 0.04
0.74 ± 0.03 0.83 ± 0.04 0.91 ± 0.03 1.01 ± 0.03 1.05 ± 0.03 1.13 ± 0.04 1.25 ± 0.05
1.72 ± 0.05 1.71 ± 0.04 1.72 ± 0.03 1.70 ± 0.02 1.70 ± 0.02 1.71 ± 0.03 1.68 ± 0.05
0.75 ± 0.05 0.84 ± 0.04 0.92 ± 0.04 1.02 ± 0.03 1.07 ± 0.04 1.15 ± 0.03 1.27 ± 0.04
1.03 ± 0.04 1.03 ± 0.03 1.03 ± 0.02 1.02 ± 0.03 1.03 ± 0.02 1.03 ± 0.03 1.04 ± 0.03
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Raman spectroscopy with 441.6 nm He-Cd lasers was used to distinguish the ZnSe secondary phase which is difficult to be identified by XRD measurement. Because of the large absorption coefficient of CZTSe material [1], during Raman measurement the penetration depth of the 441.6 nm laser is only 100–200 nm. Therefore, the results of Raman measurements only show the information of surface phase composition. The Raman spectra of the CZTSe films with different ratios of Zn/Sn are shown in Fig. 2. The Raman peaks around 170 cm−1, 196 cm−1, and 230 cm−1 belong to the B mode, A mode, and E mode of CZTSe, respectively, while the Raman peak around 250 cm−1 belongs to the main Raman peak of ZnSe [14,15]. The ZnSe signal can be identified in the samples with Zn/Sn > 1, and their intensities increase as the ratio of Zn/Sn increases. These results indicate that the ZnSe secondary phase should exist on the surface of these samples. Meaning while, weak signals near 140 cm−1 belonging to SnSe are found in these samples with Zn/Sn < 1 [16], thus identifying the existence of small amount of SnSe secondary phase in these samples. Combining above results, and considering that the 112 diffraction peaks of Zn-rich samples should also have contributions of ZnSe phase, the decrease of 112 peak intensity indicates the crystallinity is getting worse when the ratio of Zn/Sn is larger than 1.0. 3.2. Comparison of device performances In order to obtain convincing statistics data of the photovoltaic performance of the CZTSe solar cells with different ratios of Zn/Sn for comparison, we have fabricated more than 30 solar cells from 3 to 4 samples for each group with the same ratio of Zn/Sn. Each 4×4 cm2 sample produces 16 solar cells. However, because of the edge effect, the performance of 4–6 solar cells located at the four corners of substrates is always very bad compared to the other solar cells of this sample. Therefore, we omitted these solar cells from each sample during the statistics. More solar cells with Zn-rich CZTSe films were fabricated than those with Zn-poor CZTSe films to verify if Zn/Sn=1.2–1.3 is the optimal ratio for the high-performance CZTSe solar cells. The selenization process and the device fabrication processes of all the solar cells are the same. The cross section images of the CZTSe solar cells with different ratios of Zn/Sn are shown in Fig. 3. The absorbers mainly consist of large and compact CZTSe crystals on the top. The thickness of the large grains layer is around 2 µm. There are also fine grains at the bottom of CZTSe layer. Fig. 4(a) and (b) present the J-V and EQE data of the representative CZTSe solar cells with different ratios of Zn/Sn in the absorber layer respectively. The detailed photovoltaic parameters of these solar cells are listed in Table S1. Obviously, the open circuit Voltage (VOC) of these solar cells increases with the ratio of Zn/Sn, as shown in Fig. 4(a). The VOC of the CZTSe solar cells with Zn-poor absorber is less than 400 mV, while the VOC of the solar cell with Zn/ Sn=1.02 reaches 424 mV, and the Zn-rich solar cells have even higher VOC. The highest VOC of the CZTSe solar cell reaches 478 mV with the ratio of Zn/Sn=1.17. However, the VOC of the cell with the highest ratio of Zn/Sn=1.27 drops to 412 mV. On the other hand, the short-circuit current density (JSC) of the solar cells with Zn/Sn≤1 is larger than that of the solar cells with Zn/Sn > 1, which is also confirmed by the EQE data of these solar cells (Fig. 4(b)). As shown in Fig. 4(c), the band gaps of the absorber layers derived from these EQE data are in a narrow range from 1.010 to 1.045 eV. It shows the band gaps of the CZTSe layers with Zn/Sn < 1 are close to 1.01 eV, while the band gaps of the CZTSe layers with Zn/Sn > 1 are close to 1.04 eV. The small variation of band gap may result from the variation of lattice constants of CZTSe as the ratio of Zn/Sn increases [17]. The increased band gap of the Znrich CZTSe solar cells may contribute to the higher VOC of these solar cells. However, the maximum increment of band gap is only 35 meV, which is much smaller than the maximum increment of VOC (138 mV). It indicates that the variation of band gap is not the dominant factor that influences the VOC of CZTSe solar cells. The statistics data of efficiency, VOC, JSC, and fill factor are shown in
Fig. 1. (a): The XRD patterns of the CZTSe films with different ratios of Zn/Sn, and (b): The intensity of 112 peaks and the ratio of 112/220 intensity of the CZTSe films with different ratios of Zn/Sn.
Fig. 2. The Raman spectra of the CZTSe films with different ratios of Zn/Sn. The intensities are normalized to the peaks of 196 cm−1. The red arrow indicates the changes of the intensities of B mode of CZTSe, while the blue arrows indicate the change of the intensities of E mode and Sn-Se like mode of CZTSe as the ratio of Zn/Sn increases.
CZTSe films. The intensity of 112 diffraction peaks decreases slightly with the increase of Zn/Sn when the ratio of Zn/Sn is over 0.92. However, the ratio of 112/220 intensity does not change obviously with the ratio of Zn/Sn, indicating the ratio of Zn/Sn does not have a significant influence on the preferred orientation of the CZTSe films. 449
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Fig. 3. (a), (b),(c),(d),(e),(f): The cross section SEM images of the CZTSe solar cells with different ratios of Zn/Sn in absorber layer. (1) (1) (2) is the VBO of ZnSe/ ΔEVBM = ΔEVBM + ΔEVBM = 0.78 eV, where ΔEVBM (2) CuInSe2, and ΔEVBM is the VBO of CuInSe2/CZTSe. These results indicate that both the conduction band offset and valence band offset between ZnSe and CZTSe are too large that ZnSe phase can severely block both the transport of the electron current from CZTSe absorber layer to ZnO front contact and the hole current from CZTSe absorber layer to Mo back contact. TEM and EDS measurements were used to locate the position and determine the size of ZnSe clusters in the Znrich solar cells. Fig. 6 presents the TEM image of the cross-section of the solar cell with Zn/Sn=1.27. The EDS spectra of the regions labeled by red numbers are shown in Fig. S1, and the atomic percentage of the elements of the corresponding EDS spectrum is listed in Table S3. The results indicate that the large crystals near the ZnO layer (region 1) are CZTSe (region 2), and the thin layer near Mo layer is MoSex (region 4). Large ZnSe clusters with a size of about 200 nm locate at the region near the back contact (region 3). There are also lots of small grains and voids in this region. Combining with the results of surface Raman measurements, it is indicated that the poor fill factor of Zn-rich CZTSe solar cells should be attributed to the ZnSe secondary phase at the surface and the bottom of CZTSe layer. The large barrier caused by ZnSe phase may also deteriorate the JSC of the Zn-rich CZTSe solar cells.
Fig. 5(a), (b), (c), and (d), respectively. The mean value and standard deviation of the photovoltaic parameters of these solar cells are listed in Table S2. Fig. 5(a) clearly shows that the mean efficiency of the solar cells increases with the increase of the ratio of Zn/Sn at the Zn-poor region. However, the mean efficiency decreases dramatically at the Znrich region. As the ratio of Zn/Sn is 1.02, the CZTSe solar cells achieve the highest mean efficiency of 9.11%. However, the solar cells with the highest mean VOC (438 mV) are fabricated based on the absorber with Zn/Sn=1.07, as shown in Fig. 5(b). The average JSC is fundamentally invariant at the Zn-poor region and at Zn/Sn=1.02, while it decreases rapidly with the increase of Zn content at the Zn-rich region. The variation of fill factor is basically similar with that of efficiency. The poor fill factor of the Zn-poor CZTSe solar cells should be attributed to the SnSe secondary phase on the surface of CZTSe layer as shown by the results of Raman measurements. The highest mean fill factor is 63.4% at Zn/Sn=1.02, while the fill factor drops dramatically at the Znrich region. Thus the mean fill factor of the CZTSe solar cell with Zn/ Sn=1.27 is only 20.4%, leading to poor efficiencies. It should be noted that all the photovoltaic parameters of the Zn-rich solar cells show larger standard deviation than those of the solar cells with Zn/Sn≤1.02, which implies that the uniformity and reproducibility becomes worse for the Zn-rich samples. The decrease of fill factor of the Zn-rich solar cells may be caused by the high resistant ZnSe phase at the surface or the bottom of CZTSe absorber. EBIC measurement and quantification analysis suggested that the surface ZnSe acts as a current blocking phase in CZTSe solar cells [18,19]. Furtherly, we can primarily analyze the transport of electron and hole current from CZTSe layer to ZnSe phase by studying the conduction band offset (CBO) and valence band offset (VBO) of ZnSe/CZTSe interface. First-principles calculations on band structure and in suit photoemission measurements have revealed that the CBO and VBO of ZnSe/CuInSe2 hetero-junction are 0.90 eV and 0.70 eV respectively [20,21]. The band structure calculation by Chen et al. suggested that the CBO and VBO of CuInSe2/CZTSe are −0.04 eV and +0.08 eV, respectively [22]. Therefore, the CBO of ZnSe/CZTSe inter(1) (2) (1) face is given by ΔECBM = ΔECBM is the + ΔECBM = 0.86 eV, where ΔECBM (2) CBO of ZnSe/CuInSe2, and ΔECBM is the CBO of CuInSe2/CZTSe; the VBO of ZnSe/CZTSe interface is given by
3.3. Carrier density and simulations C-V measurements are carried out to characterize the carrier density and depletion region width of the CZTSe solar cells. Because a relatively high frequency AC excitation (100 kHz) was used during CV measurements for CZTSe solar cells, the defects and interface states with low escaping frequency does not respond to the high frequency excitation [23,24], and the carrier densities from C-V measurements at 0 V bias mainly represent the responses of free carriers. As shown in our previous work, the carrier densities measured from C-V and drive level capacity voltage (DLCP) measurements are very close when 100 kHz AC excitation was used during the measurements [25]. Fig. 7(a) shows the plots of carrier density (NCV) vs. distance to the junction interface, which are extracted from C-V data. Fig. 7(b) presents the statistics data of the carrier density and depletion region 450
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respectively. The QE and JSC decrease as NA increases, which should be attributed to the shrunken depletion region width as shown in the simulated band diagrams of these solar cells (Figure S2). Because the minority carrier life times (τ) of the CZTSe solar cells fabricated by the same sputtering and selenization processes in this work are in a small range of 1–2 ns [12], the minority carrier life time should have less influence on the collection of photo-generated carriers than the drastic changed depletion region width. However, the VOC increases with NA. As shown in Figure S2, the quasi-fermi level of holes (Efp) of these solar cells moves toward EV of CZTSe layer as NA increases. Therefore, the value of Efn- Efp increases with NA, which facilitates the charge separation and leads to the increase of VOC. However, the narrow depletion region width caused by the too high carrier density may lead to severe tunnel enhanced interface recombination, thus deteriorating the VOC of the device. This may be the reason why the average VOC decreases when the ratio of Zn/Sn is higher than 1.07. The simulated photovoltaic parameters for these three solar cells are listed in Table 2. The solar cell with moderate NA (1×1016 cm−3) achieves best efficiency, which is in good accordance with the experimental statistics results in Fig. 5(a). 3.4. Defects and defects clusters According to the study by Dimitrievska et al., the changes in intensities of some modes of Raman spectra reveal the information about the population changes of most dominant charge compensated defect clusters in CZTSe, namely, [VCu+ZnCu] and [2CuZn+SnZn] [27]. As shown in Fig. 2, the intensity of B modes around 170 cm−1 with respect to the A mode at 196 cm−1 decreases obviously as the ratio of Zn/Sn increases, indicating the decrease of Cu/Zn and Cu/Sn vibration units [17,27]. The decrease of B mode intensity implies that the concentration of [VCu+ZnCu] defect clusters increases with the ratio of Zn/Sn. Meanwhile, the relative intensities around 185 cm−1 and 230 cm−1 (E mode) also decrease pronouncedly with the increase of Zn/Sn ratio. The decreased intensity of Raman spectra around 185 cm−1 indicates the decreased concentration of Sn-Se vibration unites, while the decreased intensity at the region around 230 cm−1 implies the decrease of SnZn anti-sites [27]. Though the formation energy of SnZn point defect is very high, by forming [2CuZn+SnZn] defect clusters which have very low formation energy, the possibility of existence for SnZn anti-sites will increase significantly, especially in a Sn-rich and Zn-poor composition [6]. In summary, according to the changes in the intensities of B mode around 170 cm−1 and E mode around 230 cm−1, it is revealed that the population of benign [VCu+ZnCu] defect clusters increases and the population of detrimental [2CuZn+SnZn] defect clusters decreases as the ratio of Zn/Sn increases in the range from 0.75 to about 1.27. The high relative intensities of E mode around 230 cm−1 of the Zn-poor samples with Zn/Sn=0.76 and 0.84 imply that high population of [2CuZn+SnZn] defect clusters may exist in these samples. Generally speaking, these variations of [VCu+ZnCu] and [2CuZn+SnZn] defect clusters are beneficial to reduce the band tail states and improve the VOC [6,28]. Admittance spectroscopy (AS) measurements were carried out to investigate the impact of the ratio of Zn/Sn on the defect energy level and the defect density in the CZTSe solar cells. The temperaturedependent capacity frequency (Cf-T) spectra (admittance spectra) in a temperature range from 90 K to 300 K of three solar cells with Zn/ Sn=0.78 (Cell-A), 1.03 (Cell-B), and 1.12 (Cell-C) are shown in Fig. 9(a), (b), and (c), respectively. The composition of CZTSe films and the photovoltaic parameters of these solar cells are listed in Table 3. The Zn-rich Cell-C has the highest VOC but the lowest JSC and FF. Cell-B which is with Zn/Sn=1.03 has the moderate VOC and highest JSC and FF, achieving the highest efficiency among these three cells. The Arrhenius plots for the inflection points in the admittance spectra of three cells at different temperature are shown in Fig. 10. The inflection point frequency ω0 for each C-f-T curve is determined from
Fig. 4. The J-V curves (a) and EQE responses (b) of the representative CZTSe solar cells with different ratios of Zn/Sn. (c): The band gaps of the CZTSe layers derived from the EQE data in (b).
width of the solar cells with different ratios of Zn/Sn. Obviously, the carrier density increases with the ratio of Zn/Sn in the whole region from Zn/Sn=0.75 to Zn/Sn=1.27, which consolidates the conclusion of our previous work [25]. In the Zn-poor region, the carrier density is in the range of 1×1015 cm−3–5×1015 cm−3, while in the Zn-rich region, the carrier density is in the range of 1×1016 cm−3–1×1017 cm−3. In accordance, the depletion region of the solar cells shrinks as the carrier concentration increases. The average depletion region width of the solar cells decreases from about 500–60 nm as the ratio of Zn/Sn increases from 0.75 to 1.27. The effects of free carrier concentrations on the performances of CZTSe solar cells have been simulated using the software wxAMPS [26]. The setting of the simulation is listed in Table S4. The simulated J-V and QE data of three CZTSe solar cells with NA=5×1015 cm−3, NA=1×1016 cm−3, and NA=5×1016 cm−3 are shown in Fig. 8(a) and (b), 451
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Fig. 5. The statistics data of efficiency (a), VOC (b), JSC (c), and fill factor (d) of the CZTSe solar cells with different ratios of Zn/Sn. The mean value and deviation of these data are listed in Supporting information Table S2.
During AS measurements, the deep level defects with larger activation energy will be dielectric frozen out before the shallow defects when the temperature drops to a certain value. Therefore, the Ea of deep level defects is fitted at the relatively high-temperature region, while the Ea of shallow defects is fitted at the relatively low-temperature region. The defect energy levels of the Zn-poor sample Cell-A are 123 meV and 243 meV, while the trap energy level of the Cell-B and Cell-C are 51 meV and 140 meV, 47 meV and 150 meV, respectively. According to the first principle calculation, the intrinsic point defects and charge-compensated defect clusters with lowest formation energy in the Cu-poor CZTSe equilibrium are CuZn, [2CuZn+SnZn], and [VCu+ZnCu] [3,6]. The defects with activation energy between 120 meV and 150 meV present in all the samples with different ratios of Zn/Sn. Based on the theoretical study and experimental results, these traps with activation energy in the range of 120–150 meV can be assigned as the dominant acceptor impurities CuZn anti-sites [7,29]. Regarding the formation energy and excitation energy, the shallow traps with the energy level of 51 meV and 47 meV in Cell-B and Cell-C should be attributed to VCu, which becomes the dominant free hole source when CuZn anti-sites were dielectric frozen out at the temperature region lower than 150 K. According to the model described by Kimerling [30], the capacity at high frequency is attributed to the response of free carrier density, while the capacity at low frequency is attributed to the response of the sum of free carrier density and deep traps. The distribution of the deep traps in the band gap follows the equations [23,24]:
Fig. 6. The TEM image of the cross-section of a solar cell with Zn/Sn=1.27. The regions labeled by red numbers were measured by EDS. The yellow dashed circle indicates the boundary of ZnSe nano-particle. The EDS spectra are shown in Supporting information Fig. S1. And the quantitative results are listed in Table S3.
theω at the maximum point of ωdC / dω vs. ω plots. The Arrhenius plots are linearly fitted according to the equation [23]:
⎛ −E ⎞ ω0 = 2πν0T 2 exp⎜ a ⎟ ⎝ kT ⎠
⎛ 2πν T 2 ⎞ 0 ⎟⎟ E (ω) = kT ln⎜⎜ ⎝ ω ⎠
(1)
where ω0 is the inflection point frequency, ν0 is the average escaping frequency of the traps, which is independent of temperature, Ea is the average energetic depth of the defects relative to the valence band maximum, which represents an average defect energy level in the band gap.
Nt (E (ω)) = −
Vd dC ω . . qω dω kT
(2)
(3)
where E is the energy between the trap energy and the top of the 452
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Fig. 7. (a): The plot of free carrier concentration vs. the distance to the junction for representative solar cells with different ratios of Zn/Sn. (b): The statistics of free carrier concentration and corresponding depletion region width of five solar cells for each ratio of Zn/Sn.
Fig. 8. The simulated J-V curves (a) and QE data (b) of three solar cells with different free carrier densities (NA) in CZTSe layer. Table 2 The simulated photovoltaic parameters of the solar cells with different free carrier density.
valence band, ω is the frequency, ν0 is the average escaping frequency of the traps, Vd is the contact voltage of the p-n junction which can be derived from C-V measurement, and k is the Boltzmann constant. The defect density spectra of Cell-A, Cell-B, and Cell-C which are the overlaps of the plots of Nt vs. E at each temperature are shown in Fig. 11(a), (b), and (c), respectively. The densities of the defects in the Zn-rich sample Cell-C are around the order of 1016 cm−3, while those in Cell-A and Cell-B are around 1015 cm−3. It is indicated that the Zn-rich CZTSe solar cells possess a much higher concentration of VCu and CuZn defects which act as the dominant free hole carrier sources. The shallow acceptor VCu is absent in the Zn-poor CZTSe solar cells. Instead, a deep defect level with large excitation energy emerges in the Zn-poor CZTSe solar cells. One may assign the deep level defects with excitation energy of 244 meV in the Zn-poor solar cell as VZn, which is in good accordance with the calculated result (Ea=250 meV) [3,7]. However, the formation energy of VZn point defects and VZn related defect clusters are too high to achieve a large population [6]. The research by Wei et al. also reported a value of 240 meV for the Ea of CZTSSe solar cell (Eg=1.2 eV) [31]. Their experimental and simulation results reveal that the large Ea may be determined by the Fermi-level, which is pinned by combined effects of high concentration shallow acceptors such as CuZn and deep donors such as SnCu and SnZn. Though, the formation energy of SnZn point defect is very large, the [2CuZn+SnZn] defect clusters have a large possibility to exist in Zn-poor samples, which should be attributed to the very low formation energy in Zn-poor equilibrium. It is also consolidated by the previous Raman analysis about the relatively high intensity of E mode in Zn-poor samples. The [2CuZn+SnZn] defect clusters will lower the effective conduction band and act as deep level electron traps in the devices [6], which also contribute to the low VOC of
NA (cm−3) 15
5×10 1×1016 5×1016
PCE (%)
VOC (mV)
JSC (mA/cm2)
FF (%)
8.83 9.14 8.02
363 381 415
40.5 37.8 30.4
59.9 63.5 63.4
the Zn-poor solar cells besides the low free carrier concentration. The defect energy levels of Cell-B and Cell-C are very close. Therefore, the higher VOC of the Zn-rich solar cells than the solar cells with Zn/Sn close to 1 is mainly caused by the increased free carrier concentration. 3.5. The champion solar cell Fig. 12(a) and (b) show the J-V and EQE data of the 10.21% efficiency champion solar cell in this study. This CZTSe solar cell has excellent photovoltaic parameters: VOC=426 mV, JSC=36.1 mA/cm2, and FF=66.4%. The JSC calculated from the integration of EQE is 36.5 mA/cm2. The VOC of this cell is close to that of the record 11.6% efficiency CZTSe solar cell (423 mV), while the JSC is much lower than that cell (40.6 mA/cm2) [10]. It is mainly caused by the reflection loss because no anti-reflection coating is used in this solar cell, which can be implied by the fluctuant of EQE curve. Another origin of the JSC loss is the parasitic absorption of CdS layer, which is clearly confirmed by the low EQE response at the 400–500 nm. Therefore, reducing the thickness of CdS buffer layer (eg. 25 nm) and adding an anti-reflection coating on the top of ZnO:Al window layer will improve the JSC significantly. The diode parameters including Rs, A, and J0 of this solar cell are fitted using Hegedus’ method as shown in Figure S3 [32]. 453
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Fig. 9. The admittance spectra of Cell-A (a), Cell-B (b), and Cell-C (c) in the temperature range from 90 K to 300 K and frequencies from 100 Hz to 2 MHz.
the theoretical calculated value (1.00 eV) [3].
Table 3 The photovoltaic parameters of the solar cells used for admittance spectroscopy measurement. Solar Cell
Cu/Sn
Zn/Sn
Efficiency (%)
VOC (mV)
JSC (mA/ cm2)
Fill Factor (%)
Cell-A Cell-B Cell-C
1.71 1.72 1.70
0.78 1.03 1.12
7.34 8.38 5.25
368 388 440
33.0 34.8 28.6
60.4 62.4 41.7
4. Conclusions In summary, by fine tuning the ratio of Zn/Sn of CZTSe film, we have tailored the carrier density and the defects of CZTSe solar cells successfully. It is clearly revealed that as the ratio of Zn/Sn increases the hole carrier density increases, the concentration of deep level defects decreases, the concentration of shallow acceptor defects (namely, CuZn and VCu) increases, and the ZnSe secondary phase increases. The increase of the VOC of the solar cells with ratio of Zn/Sn from 0.75 to 1.07 is caused by both the declined hole quasi-fermi level resulting from the increased free hole carrier in CZTSe layer, and the decrease of deep level electron traps caused by [2CuZn+SnZn] defect clusters according to the results of Raman and admittance spectroscopy analysis. The JSC of the CZTSe solar cells with the ratio of Zn/Sn from 1.02 to 1.27 drops dramatically because of the shrunken depletion region width and the large potential barrier caused by the ZnSe phase. This potential barrier is also the main cause for the dramatically collapsed fill factor of the Zn-rich CZTSe solar cells. The statistics data show that the CZTSe solar cells with a stoichiometric ratio of Zn/ Sn=1.02 have the best photovoltaic performance. A 10.21% efficiency solar cell is fabricated based on this ratio of Zn/Sn. Acknowledgements This work was supported by the National Natural Science Foundation of China (61274053, 51372121, 51472239, and 51572132) and YangFan Innovative & Entrepreneurial Research Team Project (2014YT02N037) and by the New & Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy (No. 20123010010130). This research
Fig. 10. The Arrhenius plots of the inflection frequencies for Cell-A, Cell-B, and Cell-C, determined from the derivative of the admittance spectra in Fig. 9.The dashed lines are the linear fit of the Arrhenius plots.
The low diode saturation current (J0=5.8×10−7 A/cm2) and ideal factor (A=1.48) indicates a low concentration of recombination centers in this solar cell. The band gap derived from EQE data is 1.03 eV, very close to
Fig. 11. The defect density spectra derived from admittance spectra in Fig. 9 using Gaussian fitting for Cell-A (a), Cell-B (b), and Cell-C (c).
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Fig. 12. The J-V (a) and EQE (b) data of the champion CZTSe solar cell fabricated based on the ratio of Zn/Sn=1.02. Inset of (b): The band gap extracted from EQE curve.
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